Infestant

Infestants are ectoplasmic demon parasites searching for a suitable host (usually a vehicle - such as tank or helicopter). In their normal form they appear as large insects that lunge at their enemies. Infestants are able to take over a number of different hosts and when they find a suitable host such as the case with the Infested Tank and Infested Helicopter (in Devil May Cry 2) a large bulging eye grows in the area where the Infestant remains. The Infestants have a strong resemblance with the Chimera Seed as they use hosts and grow stronger doing so. The Infestants themselves are common enemies, but when attached to tanks or helicopter are considered Bosses.

Infestant
Since the Infestants mainly fuse with machines they most likely cannot infest living beings or they cannot infest powerful beings (as the Infestants cannot really infest the players at all even though they can still damage them) imply that it is easier for them to take over something with no life, thus they use machines, and/or the Infestants have a certain level of knowledge regarding the weapons and machines from the Human World, which is not at all impossible since the lower ranking demons are the ones who inhabit the Human World the most within the series. However they merely appear on the player's way towards the Uroboros building (Skyscraper Sanctum), implying also that them using machines can also be the work of Arius since these military vehicles are already owned by his company Uroboros to secure their facilities and their projects, and even defend his "city" on Dumary Island.

Infested Tank
Encountered in both Dante and Lucia scenarios. With them being the Infested Tank which is a powerful but incredibly vulnerable foe that due to its size, can only rotate its cannon (which will never hit the player at close range, and if it does a direct shot is practically impossible) making them only High HP demons but defenseless (although some of them have minor demons like puias or flambats around them) that are best used for stylish grade developing and orb collection.

Infested Helicopter
The boss of Mission 5 of Dante's scenario. The Infested Helicopter or the Infested Chopper is indeed harder as it never remains still, has attacks such as gattling gun shooting and rocket launching, also as it is (by obvious reasons) flying most of the time, the player has no other option than use their firearms rather than their blades/devil arms. Even with such advantages the infested chopper (helicopter) does not have a really high amount of HP for a boss, therefore a Devil Trigger-Firearm combo with the Electric Heart should make the battle much easier.

Trivia

 * The fact that the Infestant shows on the chopper as a giant eye could be a reference to the G-virus in Resident Evil, where one of the symptoms is a eye-like tumor on the shoulder.